Solid-state image sensors have found widespread applications, most notably in digital camera systems. Generally, solid-state image sensors are composed of a matrix of photosensitive elements in series with switching and amplifying elements. The photosensitive elements may be, for example, photoreceptors, photo-diodes, phototransistors, charge-coupled devices (CCD), or the like. Each photosensitive element receives an image of a portion of a scene being imaged. A photosensitive element along with its accompanying electronics is called a picture element or pixel. The image obtaining photosensitive elements produce an electrical signal indicative of the light intensity of the image. The electrical signal of a photosensitive element is typically a current, which is proportional to the amount of electromagnetic radiation (light) falling onto that photosensitive element. Conventional image sensors are fabricated using complementary metal oxide semiconductor (CMOS) technology.
The image sensors are part of a larger optical system that typically includes single or multiple lenses that are used to form the image on the photosensitive elements. The optical system is packaged using traditional CMOS packaging technology, with the exception of using a clear glass for a top surface in order to enable the reception of light on the pixels of the image sensor. Using such a traditional CMOS packaging technology, the image sensor die is bonded to a planar metal lead frame, or other planar die housing, in a process referred to as “die attach” using an epoxy, as illustrated in FIG. 1A.
Planar packaging induces several problems for image sensors, a primary problem being optical cross talk. Optical cross talk is due to the location of color filters at some distance from the pixel surface because of intervening metal and insulation layers. Optical cross-talk may be expressed in the percent of the signal lost to a neighboring pixel, which may be a left, right, up or down neighboring pixel or even a 2nd or 3rd neighboring pixel. Depending on the f-number of the lens disposed above the color filter, the portion of the light signal lost to a neighboring pixel can be large and vary significantly.
Optical cross talk degrades color separation in an image sensor because light incident on the color filter associated with a neighboring pixel is collected by a pixel assigned to a different color. More specifically, light entering the image sensor at angles other than orthogonal passes through the color filter and is partially absorbed by a neighboring pixel rather than the pixel directly below the point of entry of light on the filter of the image sensor, as illustrated in FIG. 1B. Pixels in different regions of an image sensor die are shown in FIG. 1B: a central part of a die and edge parts of the die. Each of the regions is illustrated with two exemplary pixels: a “red” pixel and a “blue” pixel. The exemplary designators “red” and “blue” for the pixels are used to denote that the pixel is intended to receive a color (i.e., wavelength of) of light based on its corresponding filter color disposed above it. The problem of optical cross-talk is illustrated on the edge parts of the die by the receipt of blue light on the “red pixel” (on the left side edge part of FIG. 1B) and by the receipt of red light by “blue pixel” (on the right side edge part of FIG. 1B). Whereas the central part of the die may not exhibit optical cross-talk between neighboring pixels as illustrated by the receipt of red light by the “red pixel” and blue light by the “blue pixel.”
Optical cross talk results in a reduction of modulation transfer function (MTF) which is a measure of the optical resolution of the image sensor. One solution for mitigating optical cross talk is to keep the distance between the color filter and the substrate, containing the pixels, as small as possible. However, such a solution imposes constraints on the aspect ratio of the pixels, which limits the reduction in overall stack height of the interconnect layers and makes pixel scaling more difficult. Another solution to partially mitigate optical cross talk is to shift the color filters and the micro-lenses residing on top of the color filter (not illustrated in FIG. 1B) towards the center of the pixel array by an amount that is proportional to the distance from the center. However, such a solution may add design and layout complexity to the optical system. The offset of color filters and micro-lenses may also induce complexities in testing of the optical system. In order to accurately test uniformity of parameters such as fixed pattern noise (FPN) which has the effect of non-uniformity in the response of the pixels in the array, dark signal non-uniformity (DSNU), and photo response non-uniformity (PRNU), a probe card must be built to mimic the angles seen by the image sensor when coupled with a lens or multiple lens stack. One approach is to test the image sensor with a lens or lens stack which may be expensive and difficult. Another conventional approach is to mimic the lens with a “pin hole” aperture or pupil in the probe card. However, such a solution drastically lowers the incident light on the image sensor and adds complexity and cost for the probe-cards and light source.